mfd: kempld-core: Check for DMI definition before ACPI

Change the detection order to priorize DMI table entries over available
ACPI entries.

This makes it more easy for product developers to patch product specific
handling into the driver.
Furthermore it allows to simplify the implementation a bit and
especially to remove the need to force synchronous probing.

Signed-off-by: Michael Brunner <michael.brunner@kontron.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>